At present, intensive work has been made on thermosensitive transfer methods, electro-photographies and ink jet methods, etc., in color hard copy technology. The thermosensitive transfer method has many advantages in comparison with other methods because of its lower apparatus and material costs as well as easy maintenance and operation of the apparatus.
In thermosensitive transfer methods, either a thermosensitive transfer material comprising a heat fusible ink layer on a support film is heated with a thermal head to record the fused ink on a transfer sheet, or a thermosensitive transfer material comprising a sublimable dye donating layer is heated with a thermal head to sublime the color material (dye) on the transfer sheet. In the latter sublimation transfer method, since the amount of color material transfer varies with the energy input to the thermal head, gradation recording can easily be attained, as a result this is especially advantageous in full color recording of high picture quality.
However, there are many restrictions on the sublimable color materials (sublimable dyes) in that very few of them meet all the performance requirements.
Examples of performance requirements thereof are good spectral characteristics for color reproduction, easy sublimation, excellent light and heat fastness, resistant to various chemicals, less reduction in sharpness, difficulty in image retransfer, easy synthesis and easy manufacture of the thermosensitive materials (ink sheets). It is particularly demanded that cyan dyes can be developed to meet these requirements.
Various cyan dyes for thermosensitive transfer applications have been proposed. The indoaniline dyes described in the specification of JP-A-61-268493, JP-A-61-31292, and JP-A-61-35994 have relatively better performance (the term "JP-A" as used herein means an unexamined published Japanese patent application). However, they do not meet all the performance requirements of cyan dyes for thermosensitive transfer applications. For instance, these cyan dyes have insufficient spectral absorption, insufficient light and heat fastness, tendency for reduction in the sharpness of the transferred images, easy retransfer, and insufficient solubility in various solvents. Because they have optical interaction with yellow dyes or magenta dyes, the light fastness of green blue regions, and black regions of the images is inferior to the fastness of the single cyan dye regions. They interact optically with other cyan dyes and the images from their combination have inferior light fastness to that of the single dye. Other defects are: insufficient gloss or images, poor preservative property of ink sheet, poor ink dispersion properties, high cost, synthesis difficulties and raw material safety.